1. Field of the Invention
The object of the invention is improvements to metal strip treatment installations.
2. Background of the Invention
The production of strips or sheets of steel or any other metal for widely differing uses usually involves hot rolling after which the rolled strip is subjected to various mechanical, chemical or heat treatments, depending on the type of product to be produced; these treatments normally include pickling followed by cold rolling.
Pickling is necessary in order to remove the oxide coating that forms on the two faces of the strip during hot rolling and storage. Pickling is performed in continuous fashion by passing the strip successively through a series of tanks containing a pickling solution, for example, hydrochloric or sulfuric acid.
The pickling tanks are arranged in sequence and are separated from each other by a threshold, comprising, for example, a deflector roller, placed above the level of the bath, the strip thus following an undulating path, dipping into successive tanks. Drying rollers are installed where the strip leaves the last tank to prevent acid from being pulled along with the strip into the next part of the installation. Rollers or other devices forming a threshold are also placed between two successive tanks to support the strip and prevent the solution from being pulled into the following tank.
After leaving the last pickling tank, the strip passes into a rinsing tank equipped with a system for pumping and spraying rinsing liquid, followed by a system for drying the liquid remaining on the two sides of the strip by evaporation.
Cold rolling is then performed in a rolling mill comprising several stands operating in tandem. Each stand comprises a series of rolls, for example, two live rolls between which the strip passes, and which bear upon two rolls of larger diameter.
Since the solution becomes charged with oxides as pickling progresses, the pickling solution is normally passed from one tank to another in the direction opposite to that of the strip, such that the last pickling tank is fed with fresh or recycled solution, whereas the first pickling tank is connected to a pipe for evacuating the used acid solution which has successively passed through all the tanks.
The optimum acid concentration therefore depends on the position of the tank in the line, and for this reason each tank is generally connected, via a recycling circuit, to a storage tank in which the concentration of the acid can be adjusted to the desired level.
However, the temperature of the solution also tends to vary from one tank to the next since the strip enters the first tank at ambient temperature, i.e., 10.degree. to 25.degree. C., and heats up in contact with the solution.
There is therefore, at least in the first tank, an exchange of heat between the strip which heats up and the solution which, in contrast, tends to cool down.
Moreover, heat losses due to evaporation and radiation occur in all the tanks; plus, in the last tank, additional heat losses occur as a result of the addition of fresh cold solution.
The most favorable conditions for the pickling operation exist when the temperature of the strip and of the solution is in the order of 75.degree. to 85.degree. C., the optimal temperature depending, moreover, on the concentration and consequently the position of the tank.
Therefore, the solution in each of the tanks is to be reheated in order to raise and maintain it at the desired pickling temperature, taking account of the position of the tank and compensating for heat losses.
For this reason, each tank is normally associated with one or more reheating circuits connected to the storage tank from where some of the solution is taken and reinjected, after filtering and reheating, at the correct temperature. The concentration can be adjusted either on the reheating circuit or directly in the tank.
Each reheating circuit therefore comprises a circulation pump and a reheating device. This reheating device could be, e.g., a heat exchanger heated by circulation of a heat transfer fluid such as steam, or alternatively by heating elements such as electric elements.
The rinsing liquid is generally also heated and its temperature maintained in similar fashion.
This reheating of the solution consumes a large amount of energy and is therefore expensive.
Moreover, this consumption of energy is irregular in that it depends on production. Indeed, since the chemical effect of the solution is exerted on the surface of the strip, the speed of travel is normally calculated so as to give the immersion time and, thence, the pickling effect required, taking account of the length of the tanks in the installation and the properties and temperature of the pickling solution.
However, the amount of heat absorbed by the strip depends on its mass and, consequently, its thickness. The greater the thickness of the strip, the greater the heat losses, which must result in a fall in temperature and, therefore, a fall in the efficiency of the pickling process which requires that the strip be slowed down, or higher consumption of energy in order to compensate for losses and maintain the solution at the desired temperature.
The invention offers solutions to such problems using particularly simple arrangements which make it possible to reduce overall consumption of energy and to very flexibly adapt the supply of energy to variations in requirements resulting notably from changes in production conditions.
In additions, for a given speed of travel, the invention makes it possible to reduce the pickling length and, consequently, the number of tanks, or alternatively, for a given pickling length, the invention makes it possible to improve productivity.
The invention can be applied either to new installations, thereby reducing their overall dimensions, or to existing installations, thereby improving their performance.